Thermoplastics are useful in the manufacture of many articles since they can be easily molded into a variety of shapes, they form hard and durable articles, and there is excellent flexibility for designs and fabrications at a competitive cost.
Controlling the gloss level of polymeric articles is important for many applications. For some polymeric materials such as acrylics, high gloss articles can be readily obtained, but low gloss or matte articles that retain their gloss levels upon subsequent processing (e.g. thermoforming) are more difficult to obtain. Often other properties, such as surface roughness and mechanical toughness are altered significantly by the matting process.
One method for obtaining matte thermoplastic articles involves the mechanical embossing of a pattern onto an article, such as by sand blasting or surface pattern fabrication. Articles made by this approach can have a low-gloss surface but typically do not maintain their gloss levels upon subsequent processing such as thermoforming or lamination onto a substrate.
Another approach for obtaining a matte finish involves the deposition of a coating or paint onto an article. A matting agent in the coating formulation provides the low-gloss surface to the article. However, this approach is not cost effective because of the need to cure and dry the coating.
Still another approach relies upon the modification of the thermoplastic resin by the addition of inorganic or organic particle of the appropriate size and refractive index. Organic particles have been used to reduce the gloss on the surface of a thermoplastic, as described in European patent EP 1022115, U.S. Pat. No. 5,346,954, and JP 2001081266. The use of organic polymeric particles produces a matte finish, but they can be difficult to make and therefore expensive. Furthermore, the handling of fine powder of organic polymer is more likely to create conditions for dust explosions than in the case of handling some silica powders.
Inorganic fillers have also been used to incorporate a matte finish on thermoplastic articles. Unfortunately, these inorganic particles, such as, barium sulfate, calcium carbonate, titanium dioxide, silica, etc. are normally difficult to disperse evenly in polymer matrix material. They also tend to reduce greatly the light transmission of the polymer, and do not generate the desired surface texture. Additionally, the filler particles have a tendency to deteriorate physical properties of the polymer.
Fine particles (i.e. less than 15 micrometers) of a copolymer made by an emulsion polymerization process are sometimes added into a plastic matrix to generate a light diffusion effect for display and light panel applications, for example, U.S. Pat. No. 5,346,954. However, this type of product tends to soften the polymer matrix and cannot produce the textured surface so often preferred by the end user.
U.S. Pat. No. 4,906,676 describes the use of silica or glass particles in thermoplastics for an optical effect. The particles are surface treated with silane compounds to render the surface hydrophobic, and thereby improve the dispersibility of the particle in the thermoplastic matrix.
The disadvantages of current approaches to producing a matte surface on a thermoplastic include: a general loss in mechanical properties (e.g. toughness); and introduction of surface roughness. In some applications such as writing or printing onto an article, roughness can be detrimental to image transfer. By choosing particles with a mean diameter<10 microns, these 2 problems can be minimized. U.S. Pat. No. 6,524,694 and Patent application Number 20030175499 describe the use of inorganic particles, such as boron nitride and silicates in extruded thermoplastics. However, in order to obtain good dispersibility a dispersing agent is required and low loading of from 0.01 to 15 weight percent. The presence of a dispersing agent can decrease the mechanical properties of the thermoplastic.
Inorganic particles with diameter<1 micron, such as micronized silica, often cause fouling of the melt-processing equipment by accumulation of deposits. Therefore, an optimum range of mean particle diameter exists in order to obtain low gloss and sufficiently low-roughness.
There is a need for a method of providing an advantageous balance of low-gloss, printability, and low migration or “plate-out”, without the need for added dispersants or particle surface coatings which add expense and may negatively effect the mechanical properties of the thermoplastic. Presently, there is not a successful plastic product in this market that can be used in a one-step process and provide the properties and appearance needed to replace sand blasted glass material.
Surprisingly it has been found that adding inorganic alkali alumina silica particles, with particle size from 2 to 20 microns, and having a narrow particle size distribution produces a matte finish, without surface roughness. Melt processing or forming (extrusion, lamination, thermoforming) of such materials yields a printable article (film, sheet, laminate) with a low-gloss appearance. The use of silica particles having the particle size and particle size distribution of the invention allows for a higher loading of particles, without the need for a surface treatment or added dispersants.